BEDFORD, Va. (AP) -- A sex offender told investigators he left a Maryland shopping mall with two young sisters who disappeared in 1975 and later saw his uncle sexually assaulting one of the girls, according to newly unsealed police affidavits.

Authorities have been searching for the remains of 12-year-old Sheila Lyon and 10-year-old Katharine Lyon on a remote mountain in Bedford County, Virginia, some 200 miles from their Montgomery County, Maryland, home, since September. A Bedford County grand jury also is investigating.

The Washington Post (http://wapo.st/19Qqopa ) reported Saturday that Montgomery County police, citing the ongoing investigation, declined to comment on the affidavits or how credible they find 58-year-old Lloyd Welch's recent statements. Police have named Welch and his uncle, 69-year-old Richard Welch, "persons of interest" in the case, but neither has been charged.

In an earlier letter to the newspaper, Lloyd Welch said he had nothing to do with the girls' disappearance. Richard Welch has declined to comment.

"Our department and our partners in Virginia remain committed to determining what happened to Katherine and Sheila Lyon," Montgomery Police Chief Tom Manger said Friday. "We believe that there are people, including family members of Dick and Lloyd Welch, who have information that would further this investigation."

Richard Welch's wife, Patricia, was charged with perjury after testifying before the grand jury in December.

The Lyon sisters walked to the mall in Wheaton on March 25, 1975, and never came home. A massive search turned up nothing.

Over four decades, police have pursued numerous leads and periodically identified potential suspects. But they never charged anyone.

The affidavits do not say whether any incriminating evidence has been found on Taylors Mountain in Virginia. Investigators have found bones on the property, according to court papers, but that may not mean much because the land contains an old cemetery with more than 30 unmarked graves. At one point during the search, detectives wrote, a dog trained to sniff decomposition "alerted" to areas away from the cemetery.

The filings detail for the first time how Lloyd Welch surfaced in the case. A week after the girls vanished the then-18-year-old Welch went to Wheaton Plaza and told a security guard that he had seen the sisters leave with a man in a car. Police gave him a polygraph test which showed he was lying, according to the affidavits, which did not say whether authorities continued to look into Welch.

The detectives on the case in 2013 set out to learn more about Welch, who had been arrested in 1977 in Montgomery in a burglary case. His mug shot from that arrest strongly resembled a sketch in the Lyon sisters case that had been drawn based on a witness's description of a man at Wheaton Plaza who was staring at the girls and following them, according to the affidavits.

Investigators also found that Welch had worked as a carnival-ride operator, traveling across the country. In 1994, he had pleaded guilty to sexually assaulting a 10-year-old girl in South Carolina. In 1998, he pleaded guilty to sexually assaulting a 10-year-old girl in Delaware and was sentenced to three decades in prison. The detectives went to Delaware, according to the affidavits, and interviewed Welch several times in prison.

"During these interviews, Lloyd Welch has admitted he left Wheaton Plaza in a vehicle with the Lyon sisters on the day they disappeared," according to the affidavits, written by Montgomery Detective Mark Janney and Detectives R.D. Baldwin and S.O. Smith of the Bedford County Sheriff's Office.

The affidavits say Lloyd Welch told police Richard Welch was involved in kidnapping the girls. Lloyd Welch said he was dropped off near his home, and the next day he went to Richard Welch's home where he saw the uncle sexually abusing one of the sisters. He said he left and never saw the girls again.